The global positioning satellite (GPS) capability of cells phones provides the opportunity to monitor location of cell phones. Services like instamapper service, available at http://www.instamapper.com/, allows a GPS-enabled cell phone to be tracked online in real time.
It is not necessary for a cell phone to have GPS capability to be monitored. Location of cell phones can also be estimated using “cell tower triangulation.” In this method, a ping can be sent from each cell tower currently detecting the signal of a cell phone. The distance of the cell phone to a cell tower can be estimated based on the lag time between the time when the cell tower sends a ping to the cell phone and a time when the cell tower receives an answering ping back from the cell phone.
Assuming a relatively flat topography, If there are at least three cell towers within range of a cell phone, and the distance between the cell and each of the three towers is known (at least approximately) from estimates based on ping lag time, then it is possible to calculate (or at least estimate) a location of the cell phone based on the location of the three cell towers.